A study is in progress to define the genetic organization of the chromosome of Staphylococcus aureus. The basic approach has been, and continues to be, multi-factorial genetic crosses performed by genetic transformation. The majority of th strains under- study become competent for transformation in the presence of 0.1M CaCl2, while a few require that competence be provoked by artificial means. The markers being used are largely auxotrophic and those responsible for resistance to antibiotics. Among the latter is Tn551, a transposon that occupies a variety of sites on the chromosome, and appears to be sufficiently stable to be useful for mapping purposes. Quite recently, additional mutants have been isolated that are auxotrophic because of Tn551 insertion (presumably into the gene(s) involved in the biosynthesis of various nutrients). Three "major" linkage groups have been defined thus far, and means are being sought to identify the relationships among these on the chromosome. Recently it has been recognized that S. aureus 8325 (the strain being employed in the studies outlined above) is highly amenable to analysis by protoplast fusion. At this writing, the optimum conditions for attaining fusion and recovering genetic recombinants have been defined. Studies are just now being initiated to determine the linear relationship of the linkage groups already defined by transformation. The nature of the competence-conferring activity observed in lysates of serological group B phage has also been examined. Thus far, it appears that the activity is due to a particle, or population of particles, that resemble empty PFU. Studies are continuing - largely by separation techniques employing centrifugation - to further purify the competence conferring activity and analyze its nature relative to the PFU.